Formatted Title

Sea Level Rise Vulnerability Assessments: An Emerging Issue at Cleanup Sites

Background/Objectives

Sea level rise (SLR) projections for the San Fransisco Bay Area in California from the Ocean Protection Council's 2018 Guidance (https://www.coastal.ca.gov/climate/slrguidance.html) estimate that by year 2100, the most likely (66% probability) value is a rise of approximately 3 feet. SLR vulnerability assessments have traditionally focused on assets/infrastructure. This presentation focuses on the application of predicted future groundwater rise to vulnerability assessments of cleanup sites in coastal counties near the San Francisco Bay. The study explores the assumptions and limitations of published model results and examines methods to enhance the adaptability of existing model results to study case scenarios, thus improving the accuracy and suitability of the published reginal models for their use at environmental cleanup sites.

Arcadis’ client leased a portion of a property adjacent to the San Fransisco Bay coastline for the operation of truck fueling and loading operations and formerly operated underground storage tanks (USTs). During UST site closure negotiations, the regulator inquired about a SLR vulnerability assessment report that had been recently conducted by the property owner for its infrastructure. The report showed that property was vulnerable to coastal flooding due to a combination of local shoreline inundation and storm surge. However, the property owner’s SLR vulnerability assessment contained various disclaimers regarding the use and reliability of the assessment and did not account for flooding from other sources, erosion, subsidence, future construction or shoreline protection upgrades, or other changes to the region that may occur in response to sea level rise. In addition, the assessment did not fully consider the existing pumps and drainage system that may reduce impacts from sea level rise, tides, and storms. Most importantly, the scenarios assessed did not include groundwater table rise evaluations/scenarios as part of the assessment. Arcadis conducted a supplemental vulnerability assessment for the UST area; specifically, to assess the potential for groundwater rise and potential diesel product remobilization and surfacing due to SLR.

Approach/Activities

The supplemental assessment of the UST area used two approaches: (1) conventional flux-controlled approach whereby a one-foot increase in SLR results in a one foot increase in water table (2) predicted water table surfacing using a publicly available  published reginal model. 

For the conventional flux-controlled approach, three wells with relatively high historical free product thickness measurements were selected, one well from each of the three distinct areas showing measurable free product in the vicinity of the former USTs. As a conservative approach, the vulnerability was assessed using the lowest of the monthly depth to groundwater/product measurements recorded during the 2021 rainy season. Predicted water table surfacing conditions assuming a conservative 1:1 ratio of SLR to water table rise. The assessment of plume vulnerability using the conventional flux-based approach and site-specific groundwater elevation data shows that the water table is at the surface (emergent) in the UST area once the SLR reaches approximately 4 feet.

For the use of published water table surfacing maps approach, the interactive mapping tool publicly available at: https://ourcoastourfuture.org/hazard-map/ was used to access predicted groundwater table rise maps generated by Befus et al. (2020). The mapping tool was set to assess the “groundwater” scenario and a conservative “low-permeability and shallow water table”. The predicted sea level rise scenario was toggled upwards starting from 0 cm until the UST Area showed the water table at the surface (i.e., emergent). The Befus et al. (2020) model shows that the plume is vulnerable to surfacing once the SLR reaches approximately 4.1 feet. In other words, the model shows that the water table will remain below the ground surface for SLR scenarios of 0 cm to less than 125 cm (approximately 4.1 feet).

Results/Lessons Learned

Our case study showed that the projected SLR of 3 feet by 2100 is less than plume vulnerability threshold of approximately 4 feet predicted by both approaches. Therefore, we reasonably conclude that because the residual diesel plume is attenuating over time and should be attenuated by 2100, the residual LNAPL plume is not vulnerable to SLR. This assessment was presented to the regulatory agency and the UST case is moving towards site closure. 

In general, groundwater emergence and surface water flooding are major risk factors at cleanup sites near the coast and groundwater fluctuations and flash flooding are the major risk factors at inland sites. Groundwater elevation fluctuation is limited by topography and soil permeability. Flooding of surface water is more directly affected by extreme weather events like coastal storms or intensive rainfalls. Both inundation risks will be increased because of SLR. Environmental cleanup sites with engineered containment systems, such as leachate collection and ancillary storm drainage systems, can be vulnerable in the near or long term.  In this case study, the published models provide useful risk determination and a sound base for the vulnerability assessment. However, more complex coastal sites may require site-specific modeling.